- Windows fog up all the time, and for a variety of reasons. Why? It all has to do with the concepts of water vapor and condensation. Sometimes windows get fogged from the inside-out and other times they get fogged from the outside-in. Either way, however, the same root cause applies. A window fogs up when it becomes colder than the dew point of the air on either side of it.
- Windows get fogged from the inside when water vapor from the air inside becomes so condensed that the temperature of the windows around the air reached that air's dew point. When this happens it causes the water vapor to condense onto the cool glass of the window. A perfect example of this scenario is a car that is parked outside on a cold day with a passenger inside. As the passenger exhales her breath, she releases water vapor into the air. This causes the humidity to rise inside the car. As the humidity of the air rises, the dew point lowers. Once the dew point of the air reaches the temperature of the car's windows, the water vapor will begin to condense onto the windows, creating a visible "fogged" appearance on the windows.
- Windows get fogged from the outside in a very similar process, simply in reverse. When the dew point of the air outside of a window is at or below the temperature of that window's glass, then the water vapor in the air will condense onto the window. This, once again, creates a "fogged" appearance on the surface of the glass from the fine droplets of water that have formed there. This type of window fogging generally happens when the air temperature outside is warmer than the temperature of the glass of the window.
